People who are hearing-impaired or unable to speak communicate via the telephone network by using specialized text terminals, commonly referred to as a telecommunication device for the deaf (TDD) or a teletypewriter (TTY). A TTY converts symbols (letters, numbers, punctuation, etc.) typed on its keyboard into audio signals that it transmits through the telephone network to another TTY. The receiving TTY converts the audio signals back into symbols and displays them on its display screen. A call between TTYs is established and terminated in the same manner as between telephones: either the TTY emulates the call signaling of a telephone, or the TTY user uses a standard telephone to initiate, dial, answer, and terminate the call, and uses the TTY only for the traffic-exchange portion of the call. The TTY usually connects to the same analog RJ-11 line as the telephone via an RJ-11 splitter.
In a Voice over Internet Protocol (VoIP) network, a SIP (Session Initiation Protocol) network, or an H.323 network, IP endpoints negotiate communication call-path parameters between themselves as part of the call set-up process prior to establishing the communication path. This is necessary in packet systems because, unlike analog telephones on the public network, IP endpoints can generate many different types of specialized packets that vary depending on the media type. Examples include voice packets, text packets, and video packets. As a part of the call set-up process, endpoints specify the types of packets that will be supported during the call. For calls between an analog endpoint on the public network and an IP endpoint, a similar call set-up negotiation occurs between the IP endpoint and the associated IP/analog gateway.
Looking now at the issue of text transmissions, analog text terminals such as TTYs do not emit a self-identifying handshake tone or rely on a carrier tone. (In this sense, they are different from computer modems and fax machines.) The devices are silent when not transmitting text. A complicating factor when attempting to use these devices in conjunction with IP networks is that, when the user is typing a message, the audio tones emitted by these devices are often not transmitted reliably by the voice-optimized audio channels of IP systems. Packet loss and audio comprehension levels commonly seen in IP networks can distort the TTY signals so badly that they cannot be decoded. For this reason, it is often necessary to transmit text via specialized non-audio mechanisms, such as the RFC-2833 protocol (“RTP Payload for DTMF Digits, Telephone Tones and Telephony Signals”) or the RFC-4103 protocol “RTP Payload for Text Conversation”).
The RFC-2833 approach transports text via IP by sending what are, in essence, verbal descriptions of the corresponding analog Baudot TTY tones. The RFC-4103 approach transports T.140-encoded text as a real-time media stream in parallel with the voice stream. Although both RFC-2833 and RFC-4103 have been demonstrated to provide reliable transport of text on IP networks, a problem remains:
Because analog TTY devices are not self-identifying, if a call is placed by a TTY user on the analog public network to the user of an IP telephone, the call set-up negotiation between the IP endpoint and the associated IP/analog gateway is likely to assume erroneously that a non-text voice-only media stream and a non-text voice-only IP endpoint will be satisfactory. The result will be, that the user of the IP endpoint will not know that he or she has received a call from a TTY user (because distorted TTY tones will be audible on the handset), but will be unable to respond because the IP endpoint is unable to receive or transmit text packets.
At least three techniques exist for providing TTY-type functionality on IP endpoints. One manufacturer has demonstrated a telephone that allows a keyboard to be connected thereto via a USB port; the keyboard is used to transmit text, and the phone's display is used to display incoming text. Others have demonstrated a soft TTY—essentially a soft phone that emulates a TTY. A third manufacturer's phone, while having no inherent text capability, allows users in telecommuter mode to set-up and control calls for which the endpoint is a traditional analog TTY device. The first and last configurations are regarded by customers as expensive and clumsy because they require additional hardware. And the chief limitation of the second configurations is that it is not supported on physical telephones.
The preceding discussion illustrates a general problem in networks: during call set-up, they negotiate (e.g., via SIP) the capabilities that the call endpoints must have in order to engage in the call. These capabilities include the medium or media that the endpoints must be able to communicate in, such as audio, text, or video. If one of the endpoints should deviate from the negotiated capabilities—by introducing a new medium into the call, for example—the other endpoint may not have the corresponding capability and thus may not be able to communicate via the call.